Immune regulation is critical in health and disease. Nowhere is this clearer than following allogeneic hematopoietic cell transplantation where dysregulated immune responses result in graft-versus-host disease (GVHD) and effective immune recognition results in control of the underlying disease, termed graft-versus- tumor effects. In this proposal, we will study the key regulators of immune reactions, namely CD4+CD25+CD127loFoxP3+ regulatory T cells (Treg) which have been demonstrated to have profound effects on control of GVHD in murine models, yet allow for GVT responses. Due to the suppression of the deleterious alloimmune effects which can also impact immune tissues, the use of Treg at defined doses along with conventional CD4 and CD8 cells (Tcon) has also resulted in more effective immune recovery. Further, it has been suggested by a number of studies that Treg can also be used to treat chronic GVHD. Therefore, the aims of this proposal are to directly translate the biological concepts developed in preclinical animal studies to test te hypothesis that the adoptive transfer of highly purified Treg will result in control of GVHD, yet promote accelerated immune reconstitution, as well as treat chronic GVHD. The Proposal involves two clinical trials both utilizing highly purified populations of Treg isolated by high-sped cell sorting through IRB and IND approved clinical protocols. The studies will directly translate important biological concepts from the laboratory to the clinic. If successful the studies could have a major impact on the field of allogeneic hematopoietic cell transplantation and could also set the stage for new treatments of autoimmune disorders and induction of solid organ transplantation tolerance. PUBLIC HEALTH RELEVANCE: Regulated immune responses play a major role in a number of different disease states. It has been recognized in laboratory studies that populations of T cells have the ability to regulate these immune responses (termed regulatory T cells) and can profoundly impact these disease entities. This proposal aims to directly translate these concepts from the laboratory to the clinic by using highly purified regulatory T cells to treat patients in he setting of bone marrow transplantation. We have chosen this clinical setting since we believe it is the most direct way to demonstrate that the regulatory cells can improve transplantation which is an important treatment modality for patients with hematologic malignancies and certain genetic disorders. If successful, these studies could improve bone marrow transplantation and open the door to more general use of regulatory T cell populations for the treatment of a variety of diseases.